Controlling an operation of a hydraulic output device of a hydraulic circuit may be conventionally accomplished using a single spool-type valve. A single spool valve has a series of metering slots which control flows of hydraulic fluid in the hydraulic circuit, including a flow from a pump to the hydraulic output device and a flow from the hydraulic output device to a tank, drain or reservoir. When the hydraulic output device is a hydraulic cylinder, these flows are commonly referred to as pump-to-cylinder flow and cylinder-to-tank flow, respectively.
The metering slots are machined into the stem of the spool valve. With this arrangement, slot timing and modulation are fixed. In order to modify the performance of the hydraulic circuit, the stem must be re-machined, which may be costly. Furthermore, in order to add additional features to the performance of the hydraulic circuit, an entirely new stem may be required. As a result, adding features to or optimizing the performance of the hydraulic circuit can be expensive and time consuming.
A more flexible system is found in an independent metering valve (IMV) assembly, which typically includes four independently operable, electronically controlled metering valves to control flows within the hydraulic circuit. The four independently controlled metering valves may be referred to as the “metering stem”. Two of the metering valves are disposed between an input port and the output ports. The other two metering valves are disposed between the output ports and the return port. Because each of the metering valves is controlled electronically, the performance of the hydraulic circuit can be modified by adjusting a control signal to one or more valves of the metering stem. Examples of IMV assemblies utilized for hydraulic functions are disclosed in US2006/0266027 and US2005/0087065.
As shown in US2006/0266027 and US2005/0087065, it is known to utilize an IMV assembly in association with an internal combustion engine. Such IMV assemblies typically receive pressurized hydraulic fluid from a hydraulic pump that is in fluid communication with a single hydraulic load providing a single hydraulic function. For example, an IMV assembly may be fluidly coupled with a two-way hydraulic cylinder used for a single output function (e.g., tipping a loader bucket on a front end loader). As an IMV assembly typically includes a metering stem with four independently controllable metering valves, one pair of valves is coupled to the head end of the hydraulic cylinder and the other pair of valves is coupled with the rod end of the cylinder. Each pair of metering valves in an IMV assembly allows flow both to and from the hydraulic cylinder. The independently controllable metering valves may be electronically controlled using a controller, typically depending upon various input signals received from one or more sensors.
Often, multiple IMVs are used in a hydraulic system that employs a single source of fluid, or a common rail. In this type of design, a single pump may pump the fluid for the common rail. One hydraulic circuit may demand more flow or more pressure than another circuit. Because a single pump delivers fluid to all circuits, there is a danger in that fluid or pressure may be delivered to a circuit at a rate or pressure that could damage the hydraulic function of the circuit. For example, excess flow or pressure to a hydraulic cylinder or to a hydraulic motor beyond the maximum capacities of these devices can cause the devices to fail.
What is needed is a system and method for controlling an IMV assembly that allows for adjacent sections of the hydraulic circuit to perform optimally without having to modify the electronic control of the metering stem. More specifically, there is a need for a way to hydro-mechanically limit the flow to a first IMV assembly when an adjacent second section of the hydraulic system demands high flow so that the high flow demanded by the second hydraulic system does not damage the hydraulic function of the first IMV assembly. Limiting flow to an IMV hydro-mechanically would be faster than relying upon the electronic control system and could possibly avoid damage to a motor or cylinder.